Special Purpose Antenna

Folded Dipole

Yagi-Uda

Log-Periodic

Turnstile

Helical

Folded Dipole Antenna1. Design

L = 143/f

where:

L is in meters

f is in MHz

Input impedance is four times higher than a basic half-wave dipoleZi = 72Ω*4 = 288Ω

To vary input impedance:Zi = Ratio*72 Ω

Where:

d1 is the conductor diameter for the feed arm of the dipole d2 is the conductor diameter for the non-fed arm of the dipole S is the distance between the conductors Ratio is the step up ratio

2. Description- a dipole antenna whose outer ends are folded back and joined together at

the center- the impedance is about 300 ohms, as compared to 72 ohms for basic half-

wave dipole3. Advantages

- wider bandwidth compare to the basic half-wave dipole- the bandwidth can be increased further by making the dipole elements

larger in diameter- well matched to 300-Ohm balanced transmission lines

4. Application- used in the simple FM dipole antennas that can be bought to use as

temporary FM broadcast antennas - used within other larger antennas such as the Yagi. - widely used with television and frequency-modulation receivers.

Yagi-Uda Antenna1. Design

Elements- The Driven Element

where feed point is located and where the feed line is attached from the transmitter to the Yagi to perform the transfer of power from the transmitter to the antenna

- Parasitic Elements element that is not directly connected to the feed line elements used for the purpose of obtaining directional power gain Reflector and Directors Reflectors

length is 5% longer than the driven element the spacing is generally between .1 and .2 wavelength Prevents antenna from sending backward

Director length is 5% shorter than the driven element the spacing is generally between .1 and .2 wavelength used to provide the antenna with directional pattern and

gain

typical spacing between elements vary from about 0.1 to 0.2 of a wavelength, depending on the specific design

elements are usually parallel in one plane, supported on a single crossbar known as a boom.

the elements must be made of some kind of conducting material: copper, aluminum, coaxial cable are best suited.

the horizontal stick (boom) can be made of anything. - if it is made of conducting material, the elements must be isolated from

the stick. Simple Three Element Yagi

- (b) is the radiation pattern

2. Description- a linear array consisting of a dipole and two or more parasitic elements:

one reflector and one or more directors- named after two Japanese scientists who invented it

3. Advantages- bandwidth can be increased by using more than one folded dipole, each

cut to a slightly different length- the typical directivity is between 7dB and 9dB- a directional antenna designed to maximize reception over long distances

4. Application- commonly used for VHF TV reception because of its wide bandwidth- used for amateur radio

Log-Periodic Antenna1. Design

Typically, , design ratio

Bandwidth:

Number of Dipoles:

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the length and spacing of the elements of a log-periodic antenna increase logarithmically from one end to the other

element at the back of the array is a half wavelength at the lowest frequency of operation

the feed phase is reversed from one element to the next to ensure that the phasing of the different elements is correct

the direction of maximum radiation is towards the feed point

2. Description- Consist of several dipoles of different length and spacing that are fed from

a single source at the small end. The transmission line is crisscrossed between the feedpoints of adjacent pairs of dipoles.

- Physical structure is repetitive, making electrical characteristics repetitive as well

- it offers less gain for its size than does the more conventional Yagi - the antenna input impedance varies repetitively when plotted as a function

of frequency and periodically when plotted against the log of the frequency

3. Advantages- independent of radiation resistance and radiation pattern to frequency- able to provide directivity and gain while being able to operate over a wide

bandwidth4. Application

- used where a wide range of frequencies is needed while still having moderate gain and directionality

- used at VHF and UHF for a variety of applications, including some uses as a television antenna

- used on the HF portion of the spectrum where operation is required on a number of frequencies to enable communication to be maintained

Turnstile Antenna1. Design

consists of two horizontal half-wave antennas mounted at right angles to each other in the same horizontal plane

Radiation Pattern - The radiation pattern shown is the sum of radiation patterns from the

two dipoles, which produces a nearly omnidirectional pattern

2. Description- a set of two dipole antennas aligned at right angles to each other and fed

90 degrees out-of-phase - when mounted horizontally the antenna is nearly omnidirectional on the

horizontal plane. - when mounted vertically the antenna is directional to a right angle to its

plane and is circularly polarized. - balanced transmission lines

3. Application- developed primarily for omnidirectional vhf communications

- often used for communication satellites because, being circularly polarized, the polarization of the signal doesn't rotate when the satellite rotates

Helical Antenna1. Design

the dimensions of the helix are determined by the wavelength λ of the radio waves used, which depends on the frequency.

the length of the coil determines how directional the antenna will be as well as its gain; longer antennas will be more sensitive in the direction in which they point.

a ground plane at the driven end makes the radiation unidirectional from the far (open) end

where: r is the radius of the helix a is the pitch angle S is the spacing between turns L is the length of one turn is the tangential unit vector describing the contour of the

helix Zo is the starting height of the helix

pitch angle α:

where:s is the spacing from turn toturn r is the radius of the helix

the ground plane diameter is typically 0.94l in diameter at the center frequency the radiating element is a helix of wire, driven at one end and radiating along the

axis of the helix

Power Gain

where:Ap(dB) = antenna power gain (dB)D = helix diameter (meters)N = number of turns (any positive integer)S = pitch(meters)λ = wavelength (meters per cycle)

3-dB Beamwidth

where:θ = beamwidth (degrees)D = helix diameter (meters)N = number of turns (any positive integer)S = pitch(meters)λ = wavelength (meters per cycle)

2. Description- An antenna that has the form of a helix- A helical antenna is a specialized antenna that emits and responds to

electromagnetic fields with rotating (circular)polarization 3. Application

- typically used for applications where reduced size is a critical operational factor

- most suitable for Mobile and Portable High-frequency (HF) communications in the 1 MHz to 30 MHz operating range

- used extensively for 27 MHz CB radio

References:

Electronics Communications System 5th Edition by Wayne Tomasi

http://www.electronics-radio.com/articles/radio/antennas/dipole/dipole-antenna.php

http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html

http://www.hamuniverse.com/yagibasics.html

http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.html#Introduction

http://www.changpuak.ch/electronics/yagi_uda_antenna.php

http://www.radio-electronics.com/info/antennas/log_p/log-periodic-array-feeder-arrangements.php

http://www.tpub.com/content/neets/14182/css/14182_211.htm

http://strobbe.eu/on7yd/136ant/